1. Field of the Invention
This invention relates to the inclusion of amino acid based monomers into polymeric compositions for biomedical uses.
2. Description of the Prior Art
Carboxylic acid containing polymers for contact lenses and other biomedical uses are well known, for instance see U.S. Pat. No. 3,822,089 granted July 2, 1974 to Otto Wichterle. Their major advantage is the enhanced water content of the polymer and higher oxygen permeability. However, these polymers have low mechanical strength. As a result, other polymeric compositions have been suggested. U.S. Pat. No. 3,937,680 granted to de Carle discloses a terpolymer of hydroxyethyl methacrylate, N-vinyl pyrrolidinone and acrylic acid. While water contents of up to 90% can be achieved, the mechanical properties are unsatisfactory. U.S. Pat. No. 3,988,274 granted Oct. 26, 1976 to E. Masuhara et al discloses that the strength of carboxylic acid containing polymers are enhanced by the use of alkyl methacrylates, e.g., n-butyl methacrylate. Further, attempts to improve polymers include the addition of another vinyl monomer, vinyl acetate, see U.S. Pat. No. 4,143,017 granted Mar. 6, 1979 to N. Tarumi et al.
Unexamined Japanese patent publication No. 149014/81 discloses an amino acid copolymer of 30% benzyl glutamate and 70% leucine for contact lenses. The polymer has a water content of 2%. After treatment with 3-hydroxypropylamine for 65 hours the water content was increased to 48% with a slight increase in oxygen permeability. This polymer is made with (1) only amino acids and (2) a different approach to obtain a polymer with entirely different characteristics than the polymers of the present invention. The post treatment of the reference polymers is unsatisfactory because of the length of time required, variations in products and side products produced. Derwent Publications abstract 96145D of unexamined Japanese patent publication No. 149015/81 discloses an amino acid copolymer of alanine and methionine (2:1) can also be used for contact lenses, but no details of preparation or properties thereof are disclosed.
Typically to fabricate a contact lens from the carboxylic polymers above, polymeric rods are cast, sliced and subsequently lathe cut. Irrespective of the final article the polymerization usually takes considerable time, up to a week, combined with controlled temperature gradients to avoid bubbles and stresses in the article.
Certain biomedical articles, e.g., contact lenses, can also be prepared by spincasting. Spincasting of these polymers using open molds is difficult because of the volatility of the monomers, e.g., acrylic or methjacrylic acid, N-vinyl pyrrolidinone, and methyl acrylate or methacrylate. During spincasting, in a thin layer at the surface, evaporation changes the composition of the original monomer mixture which in turn affects the reproducibility of the contact lens properties.
It has now been found that monomers derived from amino acids are non-volatile and very often crystalline materials which can be dissolved in water or other solvents including comonomers. These monomer solutions do not change their composition during spincasting.